The present invention relates to an improved nozzle assembly for an injection molding machine and, more particularly, to an improved sealed edge gate arrangement to be used therein.
Hot runner edge gating systems are well known in the art. When such systems are used to process heat sensitive resins however, it becomes important to minimize or, if possible, eliminate any bubble or resin insulation well so as to avoid degraded resins, trapped within the nozzle assembly and its components, from being drawn into the main melt stream and thereby into the molded part.
U.S. Pat. No. 4,344,750 to Gellert and U.S. Pat. No. 4,981,431 to Schmidt show typical non-bubble type hot runner edge gating systems. The system shown in the Gellert patent uses individual hollow seals to connect the hot runner nozzle directly to the cavity gate. This totally eliminates the bubble because the nozzle is completely surrounded by an insulating air gap. The seals used in this system are pressed into recesses in the nozzle, one seal for each gate. As the nozzle assembly is fitted into the mold cavity plate, the seals deform slightly inwardly so as to effect a mechanical seal with the cavity plate. Removal and replacement of the nozzle for servicing requires the replacement of the seals each time. Another disadvantage of this approach is that the seal is made of titanium, which although having a lower thermal conductivity than steel, still permits a significant amount of heat to be conducted from the heated nozzle to the cooled mold cavity. As a consequence, the nozzle has to be heated to a higher temperature than would otherwise be required to process the resin. Normally this is not detrimental, but when heat sensitive resins are processed, this can be troublesome since these resins easily degrade at temperatures only slightly higher than their processing temperature.
The Schmidt patent also uses individual titanium seals individually screwed into the nozzle assembly. The seal design uses a very small bubble to locally insulate the gate from the nozzle and thereby reduce the heat conducted through the seal. A disadvantage of this design is that the stiffer seal construction is less elastic and does not readily deform during installation like the Gellert seal. This means greater accuracy in manufacture and assembly are required in order to assemble the nozzle. Also, since this seal is larger than the Gellert seal, there is less space in the nozzle to accommodate multiple seals for multiple gating. The most attempted has been four. Additional orifices tend to weaken the nozzle at the tip end where strength is most important. At the tip end, the injected resin travelling at high speed and under high pressure must change direction through ninety degrees subjecting the end of the nozzle to very high stresses. The risk of blowing the end off the nozzle is increased by the addition of seals for multi-cavity gating.
Both of the sealing devices shown in the Gellert and Schmidt patents have the disadvantage that the sealing and the location of the gates is local to each gate. Thus, in a two cavity arrangement, there is a tendency for the nozzle assembly to cock or jam when being assembled or disassembled since alignment and contact with the mold cavity occurs in only two places within the mold cavity location diameter.
Accordingly, it is an object of the present invention to provide an improved sealed edge gate system for an injection molding system.
It is a further object of the present invention to provide an edge gate system as above which facilitates the processing of heat sensitive resins.
It is still a further object of the present invention to provide an edge gate system as above which provides a nozzle assembly having improved thermal insulation properties.
Other objects and advantages of the present invention will become more apparent from the following description and the accompanying drawings in which like reference numerals depict like elements.